Chemists in the field of molecular design and synthesis are devoting considerable effort towards the development of DNA-cleaving agents. For recent, representative examples, there are K. M. Hess and T. A. Dix, Anal. Biochem., 1992, 206, 309; K. C. Nicolaou, W.-M. Dai, S.-C. Tsay, V. A. Estevez and W. Wrasidlo, Science, 1992, 256, 1172; M. Sako, K. Nagai and Y. Maki, J. Chem. Soc., Chem. Commun., 1993, 750. The prior art DNA-cleaving agents are in general difficult to synthesize and thus have a high manufacturing cost. In addition, the DNA-cleavage processes of using these DNA-cleaving agents often require additional metal ions, an external sensitizer or H.sub.2 O.sub.2, and thus are somewhat complicated or not easy to controll, so that the cleavage efficiency is low and can be further improved.